1. Field of the Invention
This invention relates to a method for controlling the thickness of a single crystal thin film layer in an SOI (silicon on insulator) substrate. More particularly this invention relates to a method for controlling the thickness of a single crystal silicon layer bonded to a dielectric substrate to ensure formation of the single crystal silicon layer in the form of a thin film.
2. Description of the Prior Art
Heretofore, for the purpose of ensuring production of the single crystal silicon layer bonded to the dielectric substrate of this kind in the form of a thin layer, generally the following various means have been proposed.
As the first means for the formation of a single crystal silicon thin-film layer on a dielectric substrate, the so-colled SOS (silicon on sapphire) method, namely the technique of causing a single crystal silicon layer to be epitaxially grown on a single crystal sapphire substrate, has been well known.
This SOS method, however, is short of practicality because numerous crystal defects are liable to occur during the vapor-phase silicon growth owing to the mismatch in lattice constant between the sapphire and the single crystal of silicon in the process of vapor-phase growth.
As the second means, the technique which comprises forming a thermal oxide film on the surface of a silicon substrate and, at the same time, depositing a polycrystalline or amorphous silicon film on the thermal oxide film and then irradiating the thermal oxide film in a linear pattern with an energy beam such as an electron beam or a laser beam and, at the same time, moving the direction of this irradiation gradually in a scanning pattern in the perpendicular direction thereby fusing and solidifying the silicon layer and forming single crystal thin-film on the entire surface of the substrate has been disclosed in Japanese Patent Publciation SHO 62(1987)-34,716.
In this known technique, a single crystal projection is exposed in the terminal part of the single crystal silicon substrate and the conversion of the polycrystalline film into a single crystal is attained with the projection as a core. This technique in its existent state is sparingly capable of producing a single crystal silicon thin layer fit for practical use, though the interaction between the fused silicon and the oxide film permits the conversion into the single crystal to be effected partly.
As the third means, the so-called SIMOX (separation by implantation oxygen) method, namely the technique of implanting oxygen ions in a silicon substrate by the use of an ion-implanting device and then subjecting the silicon substrate to an annealing treatment thereby forming an oxide film layer in the part at a specific depth inside the silicon substrate has been well known.
Again this technique is hardly fit for practical use in its existent state because the crystal defects produced by the implantation of ions defy restoration.
In recent years, therefore, the wafer of the so-called bonded SOI (silicon on insulator) construction has come to attract special attention.
The wafer of this bonded SOI construction is fabricated by preparing two single crystal silicon wafers, subjecting at least either of the silicon wafers to an oxidizing treatment thereby forming an oxide film on the surface of the silicon wafer which has undergone the oxidizing treatment, superposing these two silicon wafers on each other in such a manner that the oxide film formed on at least one silicon wafer will constitute itself an intermediate layer, heating the superposed silicon wafers to a prescribed temperature thereby bonding them, and subsequently polishing the upper layer side silicon wafer into a thin film (hereinafter, the one wafer subjected to the conversion into a thin film after the bonding shall be referred to as the "bond wafer" and the other wafer as the "base wafer").
One of the single crystal silicon wafers of the bonded SOI construction is replaced by quartz glass substrate mirror finish as the base wafer and a single crystal substrate furnished with no thermal oxide film as the same on the bond wafer.
In the single crystal silicon wafer of the conventional bonded SOI construction which is fabricated as described above, various minute devices are formed by the universally known technique for production of semiconductor integrated circuit elements on the single crystal silicon layer which has undergone the conversion into the thin film. Recently, the devices so formed are incessantly tending toward reduction in size and growth in density. Further, the single crystal silicon wafers which are used for special devices such as, for example, drive circuits in displays are required to be such that the single crystal silicon layers on the dielectric substrates should have their entire surfaces finished with high accuracy On the order of submicrons, namely the dispersion of thickness of the thin film layers across the substrate should fall within .+-.10% of the average film thickness.
This is because the dispersion of thickness of the single crystal silicon layer subjected to the conversion into thin film has a serious influence of causing a dispersion in the electrical properties of the component elements to be formed within the silicon layer.
In the conventional technique for the conversion into a thin film, however, the conversion of a single crystal silicon layer into a thin film is normally attained by the work of surface grinding or the work of mirror polishing. Further, in the work for the conversion into a thin film, it is the unbonded surface of the base wafer, namely the silicon substrate or quartz glass substrate that serves as the reference plane. For the purpose of enhancing the accuracy of thickness of the thin layer, therefore, first uncontrollability of thickness of the base wafer itself poses as a problem and then the processing accuracy of surface grinding performed on the bond wafer in the technique for mirror polishing poses as another problem. The accuracy of thickness of the silicon substrate or the quartz glass substrate destined to serve as the base wafer, in the existent state, is about .+-.0.30 .mu.m at best. This point puts a serious technical obstacle in the way of producing a single crystal silicon wafer of the SOI construction furnished with a single crystal thin layer of high accuracy on the order of submicrons.
When the single crystal thin layer in the silicon wafer of the SOI construction is assumed to have an average thickness of 0.50 .mu.m, for example, the possibility that this thin layer will have the largest thickness of about 0.80 .mu.m and the smallest thickness of about 0.20 .mu.m claims a recognition. The difference, 0.60 .mu.m, between the two extreme thickness mentioned above exceeds the average thickness of the layer. When the single crystal thin layer in the silicon wafer of the SOI construction has an average thickness of not more than 0.50 .mu.m, the possibility that part of the single crystal thin layer will be completely lost during the fabrication thereof by the conventional technique is not undeniable.
This invention has been realized for the purpose of solving the problems encountered by the prior art as described above. An object of this invention is to provide a method for the production of a silicon wafer of the SOI construction comprising a dielectric substrate and a thin layer of single crystal silicon bonded to the dielectric substrate and endowed with accuracy of thickness on the order of submicrons, which method enables the fabrication to be controlled with accuracy such that the dispersion of thickness of the single crystal silicon thin layer will fall within at least .+-.10% of the average thickness and, at the same time, ensures infallible retention of the required single crystal quality in the thin layer before and after the fabrication. This method particularly is directed to controlling the thickness of the single crystal thin layer in the SOI substrate.